this is a very vague question but i'd say they are modifying their emotional state
if two animals are held in captivity seperate when they finally meet they will more than likely be hostile to each other because they dont know what a friend is lets take a cat for example dont give a cat a friend and now it sees a interesting laser on the floor and chases it well thats adorable and all but you just taught your cat how to be hostile and hunt and it will do that for the rest of its life well now lets say this cat is 2 and an adult female when she comes across another female or another species of animal she will definitely attempt to fight it because its all she knows